Security fences are often installed in facilities and areas requiring security. Also, in order to monitor an intrusion into the security fences, monitoring personnel may be deployed. However, a monitoring scheme based on monitoring personnel incurs high maintenance cost and has low monitoring accuracy.
As a complement, recently, a monitoring method using a CCTV has widely been used. This method is reduced in labor input but still disadvantageous in that a person should directly monitor a captured screen. Also, in case where a monitoring area is as large as a few kilometers or greater, a larger amount of camera devices is required to be installed, increasing cost for installation and maintenance of the devices.
Also, recently, a radio frequency (RF) scheme has been used. According to the RF scheme, a transmission line is installed in a security fence and an electrical RF signal is applied to the transmission line. If an intruder passes through or damages the transmission line, the RF signal is changed to automatically detect the intrusion. This scheme, however, has shortcomings in that a location where intrusion occurs may not be accurately specified.
An optical line monitoring method using an optical fiber solves the shortcomings. The optical line monitoring method using an optical fiber is advantageous in that a large area may be monitored, intrusion alarm may be automatically made, and a location where intrusion occurs may be specified to be recognized. However, an optical line monitoring device used for the optical line monitoring method is costly.